Master-detail relationships allow roll-up summary fields to aggregate data from child records. If a roll-up summary field exists on the Account object, the relationship cannot be changed to a lookup because lookup relationships do not support roll-up fields.
Why not other options?
A: Null values in the Vendor records do not prevent the change.
C: Custom fields can have their types changed as long as dependencies like roll-up fields are not present.
D: Organization-wide defaults do not affect the ability to change relationships.
Salesforce Master-Detail Relationships and Roll-Up Summary Fields